The invention relates to a method for evaluating an input data signal generated by load modulation, and also to a circuit arrangement for carrying out such a method.
In magnetically coupled, contactless identification systems consisting of one or more contactless data carriers and a write/read apparatus, the data transmission from the data 5 carrier to the write/read apparatus and vice versa is tested on the basis of the so-called maximum read/write distance. The maximum read/write distance offers an important criterion as regards the operation of the devices, notably when the write/read apparatus and the data carrier (carriers) are to operate only in the tested combination.
The contactless data carriers are magnetically coupled to the write/read apparatus. The energy for the power supply of the data carrier as well as the data signals themselves are transferred via the magnetic coupling. The transfer of the data signals from the data carrier to the write/read apparatus takes place via load modulation. To this end, an additional impedance, referred to as a load impedance, is switched on and off by the data signal on the data carrier. An arrangement of this kind is shown in general form in FIG. 1. An inductance 1 of the write/read apparatus and an inductance 2 of the data carrier therein constitute the arrangement for the magnetic coupling of the data signals and the power supply.
Parallel to the inductance 2 of the data carrier there is connected a first impedance 3 which serves as a fixed load, and also a load impedance 5 which can be activated via a switch 4. The switch 4 is preferably electronically controlled by the data signal to be applied from the data carrier to the write/read apparatus (see the arrow 6).
For independent development of the write/read apparatus on the one hand and the data carrier on the other hand, and for definition of the operating parameters, specification of these two components is necessary in the sense of standardization according to the ISO standard 14.443. In this context it is to be noted that the standard ISO/IEC JTC1/SC17/WG8/TF2 proposes a measuring arrangement for determining the modulation of the data carrier, by means of a coil system which is diagrammatically shown in FIG. 2. The mechanical and electrical data of this measuring arrangement are defined in the document xe2x80x9cWorking Draft ISO/124443xe2x80x9d. In the construction shown in FIG. 2 a field coil 7 generates a magnetic field having an adjustable field strength. In the absence of a data carrier this field induces equally high voltages in two measuring coils 8, 9 which are symmetrically arranged relative to the field coil 7. The two measuring coils 8, 9 are connected in series in phase opposition, so that the difference voltage between the voltages induced in the measuring coils is at least substantially zero. The difference voltage can be adjusted to a minimum value by means of an adjusting device (not shown).
When a data carrier 10 is introduced into one of the measuring coils, the load in the form of the data carrier 10 causes the voltages induced in the measuring coils 8, 9 to no longer compensate one another, so that the difference voltage indicates the loading by the data carrier 10. Load modulation of the data carrier thus provides amplitude modulation of the difference voltage. Such an amplitude modulated difference voltage is denoted by the reference UD in FIG. 2.
It has been found in practice that the evaluation of the amplitude modulation is dependent on the equalization of the measuring coils 8, 9, i.e. that it can be falsified by equalization errors. It is an object of the invention to provide a method and a circuit arrangement which enable more reliable evaluation of a data signal transmitted via load modulation.
According to the invention this object is achieved by means of a method as defined in claim 1 and a circuit arrangement as defined in claim 10. The method according to the invention and the circuit arrangement according to the invention utilize the recognition of the fact that the load modulation not only induces modulation of the amplitude, but also modulation of the phase. The invention evaluates the amplitude modulation as well as the phase modulation in common, so that improved protection against disturbances can be achieved. Moreover, for measuring purposes a plurality of transmission parameters can be simultaneously determined in a very simple manner. The detection and determination of these parameters can be carried out in automatic measuring devices.
The method according to the invention enables determination of the modulation properties on the basis of the input data signal generated by load modulation. To this end, first a complex data signal is generated from the input data signal. In an advantageous version of the invention, as disclosed in the dependent Claims, such determination can be performed by quadrature mixing and by assigning the so-called in-phase signal of a quadrature mixer used to the real part and the quadrature signal to the imaginary part of a complex envelope. For the complex data signal, then being present in the form of the complex envelope, a decision line is then determined in the complex plane, i.e. in the representation of this signal as a complex locus curve, by making the complex envelope approximate a straight line, rotating it through 90xc2x0 in the representation of the complex locus curve, and making it extent through the mean value of the complex envelope. The decision line thus divides the complex envelope into two parts for the two load states xe2x80x9cloadedxe2x80x9d and xe2x80x9cunloadedxe2x80x9d which can be distinguished in the demodulation of the input data signal generated by load modulation. Finally, the two mean values of the complex envelopes can be determined for these two load states, and the modulation properties and the input data signal can be evaluated on the basis thereof.
Instead of evaluating the complex envelope, a complex baseband signal can also be evaluated. The evaluation can also be performed by digital or analog Fourier transformation instead of quadrature mixing. It is also possible to perform sampling in the time domain instead of using a quadrature mixer, i.e. preferably by means of two sample-and-hold circuits whose sampling instants have been offset by at least approximately one quarter of the period duration of the carrier oscillation of the input data signal.
The method according to the invention and the circuit arrangement for carrying out such a method can thus be very universally used.
In the preferred versions disclosed in the dependent Claims, the amplitude swing resulting from the difference between the absolute values of the two mean values for the two states can be determined from the mean signal values, separately determined for the loaded state and the unloaded state, by evaluation of the amplitude modulation. This value, divided by the sum of the absolute values of said two mean values, determines the modulation index.
The phase modulation can be evaluated by determining the mean phase angles of the input data signal for the two states. The difference between these phase angles, i.e. the phase angle between the two complex mean values for the two states, yields the phase swing of the load modulation.
Moreover, the absolute value of the difference between the two complex mean values for the loaded state and the unloaded state can be evaluated as a complex modulation swing.
The invention is implemented preferably in an apparatus for evaluating an input data signal which is generated by a data carrier by load modulation. Such apparatus is formed by measuring equipment; however, it can also be used in a wide variety of applications in the data communication field. In practice a high transmission reliability can thus be achieved for a wide variety of applications.